It's just about everything that can be purchased online.
I own several aquariums, it's getting almost impossible to get good supplies for them as it's no longer profitable to stock them (just get them online right? But what if you need support on a $200 pump right now instead of having to wait 3 weeks for shipping while your fish die?).
As an avid amateur photographer I see camera stores going slowly under.
Just about anything can be purchased more cheaply online and people are indeed doing the same with photographic equipment as you describe about bookstores.
Except here the damage is worse and more immediate. If I want to see and compare equipment in a store I'm tying up their staff directly because they're demonstrating the stuff to me. In that interval a real customer might leave because there's noone to help them...
But it is sometimes hard I agree. With prices online being often 30-40% lower than in stores (and in severe cases over 50% lower) stores can't compete anymore.
I've spoken about this with several storeowners. They say openly that the prices they are charged by importers and manufacturers are often higher than the prices online stores charge to end users.
Add the store's markup (salestax here is included on items purchased online as well) and the situation becomes impossible for the independent store owners.
For example, at one point I was offered online an expensive lens for €1800 (including shipping).
The only local supplier after a big discount and tradein of several hundred Euro worth of old equipment was able to go down to no less than €2500.
I purchased neither item, instead opting for an alternative that the store could offer at a decent price...
With books (and especially computer books) the differences are often similar (especially with US imports where the prices in Euros often in no way reflect the dollar price).